1. Field of the Invention
The present invention relates to an automatic tuning apparatus. More specifically, the present invention relates to an improved automatic tuning apparatus using a voltage controlled variable reactance device.
2. Description of the Prior Art
In reception of the television broadcast, it is necessitated to select a desired channel out of several television broadcast channels. One of the typical channel selectors so far employed in selection of television channels is a rotary switch. However, a rotary switch usually makes a circuit through a mechanical touch between paired contacts performed by a manual rotating operation. For this reason a rotary switch always suffers fatally from a poor electrical contact between the paired contacts caused by deterioration of the contact surfaces, a tiresome manual operation and a noise caused by rotating operation thereof. Another typical channel selector suitable for selection of television channels is a multi-circuit push-button switch. Again, however, the push-button switch involves the same shortcoming of a poor electrical contact on the same ground as a rotary switch.
Recently a television channel selecting apparatus for selecting VHF television channels employing a voltage controlled variable capacitor has been proposed and will be expected to be widely used in the future. Such a voltage controlled variable capacitor uses a capacitance formed at a barrier portion of a diode, variable as a function of a reverse voltage applied thereto, and is also known as a variable capacitance diode. In such a television channel selecting apparatus, it is necessary to provide a voltage supply capable of providing a plurality of different voltages in response to manual operation each corresponding to a value which causes a respective capacitance valve in accordance with which the television channel selecting apparatus selects a desired corresponding channel. One type of such a voltage supply for providing a plurality of different voltages comprises a reference voltage source and a plurality of potential dividers for dividing the voltage of the source, each being selectively operable in response to individual manual operation. Another type of such a voltage supply for providing different voltages to a variable capacitance diode comprises a capacitor and a charging/discharging circuit therefor, a voltage across the capacitor, as selected in response to an output of the channel selecting apparatus, being applied to the variable capacitance diode. More specifically, the capacitor is charged or discharged by the charging/discharging circuit until a voltage across the capacitor causes capacitance in the diode in accordance with which the selector selects a corresponding television channel to provide a tuning output therefrom when the charging/discharging circuit is disabled and no longer continues to charge or discharge the capacitor, so that the voltage thus provided across the capacitor remains the same. If another channel is desired, the charging/discharging circuit is again enabled through manual operation and the abovementioned operation is repeated until another channel is selected. Nevertheless, the voltage across the capacitor gradually becomes lower because of leakage through an undesired leakage path, which causes detuning of the selecting apparatus and thus results in a poor quality of picture on a screen of a cathode ray tube of the television receiver. In order to solve such a problem it is required to provide a circuit for compensating the decrease in voltage across the capacitor because of leakage, which makes the circuit complicated and the system expensive. Another disadvantage in the television channel selecting apparatus using a variable capacitance diode supplied with a voltage across the capacitor is that the voltage across the capacitor is charged or discharged in an exponential manner rather than in a linear manner, which makes it difficult to achieve precise automatic tuning. It is desired that an improved voltage supply source for providing different voltages to a variable capacitance diode is provided, which is suited for employment in an automatic tuning apparatus.
A voltage storing device of interest in connection with the present invention is disclosed in U.S. Pat. No. 3,735,110, issued Aug. 14, 1973 to Hironosuke Ikeda et al. and assigned to Sanyo Electric Co., Ltd. the same assignee as that of the present invention. As set forth in the referenced patent, Professor Takehiko Takahashi and Assistant Professor Osamu Yamamoto, Technological Department of Nagoya University, announced their study on the electrochemical potential memory device by the use of a solid state electrolyte at the 22nd annual assembly of Japan Chemical Association held on Apr. 5 to 7, 1969. Briefly stated, this device comprises an Ag electrode as a cathode, an Ag--Te alloy electrode as an anode, and a solid state electrolyte having high ion conductivity, such as RbAg.sub.4 I.sub.5 sandwiched between both electrodes. When a DC voltage is applied to the device so that the Ag electrode may be negative, a portion of Ag contained in the Ag--Te alloy electrode migrates over to the Ag electrode, resulting in a decreased activity of Ag in the Ag--Te alloy, and thus an increased potential difference between both electrodes. The inventors of this device termed this states of operation as "charging". When the polarity of the applied DC voltage is reversed to that of the former case, Ag is refilled into the Ag--Te alloy, resulting in the decreased potential difference and returns to the initial value eventually. The inventors of this device termed this state of operation as "discharging". Study disclosed by the inventors of this device indicates that the electromotive force generated by the abovementioned charging or discharging current can cause linear change to some extent with respect to the charging or discharging time. Thus, this device makes it possible as an outstanding characteristic to do write-in and non-destructive read-out operation while preserving relatively linear relation between the charging or discharging time and terminal voltage, and in addition, it can hold the memory condition for a relatively longer period of time. These advantages mean that this device has opened a way for its potential use as an analogue memory device. The referenced patent further discloses an improved electrochemical potential memory device. More specifically, FIG. 6 of the referenced patent shows an improved electrochemical potential memory device for eliminating the IR drop across the resistance in the electrolyte and the overvoltage caused by dissolution or deposition of Ag, which is basically characterized by the provision of an auxiliary cathode that comprises an output terminal for detecting the potential separately from the abovementioned cathode avaiable for the input terminal for the current conduction.
In view of these advantageous characteristics of the abovementioned memory device, it may be possible to utilize this device as an essential component of a solid state variable voltage supply source. The present invention has thus been accomplished by the inventors in order that such possibility may be realized.